The core–shell magnetic nanocomposite of Fe3O4@SiO2 was synthesized by silica source that extracted from rice husk. Afterward, functionalized by 3-aminopropyltrimethoxysilane (APTMS) via post-synthesis grafting for selectivity improvement of methyl red (MR) adsorption. The magnetic property, chemical structure and morphology of the synthesized sorbent (Fe3O4@SiO2@NH2) were characterized by XRD, zeta potential, VSM, FT-IR, SEM, EDX and TEM analyses, respectively. Adsorption of MR onto the sorbent was investigated with designing 20 series of experiments by the CCD under RSM. The interaction between three separate variables including initial pH of solution, initial concentration of MR and dosage of sorbent were considered for obtain R (removal efficiency) and qe (adsorption capacity) as the responses. The optimum pH, MR concentration and sorbent dosage found 5.24, 100 mg L−1 and 0.5 g L−1, respectively, were given maximum R of 51.64% and a maximum qe of 81.39 mg g−1 with desirability of 0.826. Adsorption kinetic and isotherm studies revealed that pseudo-second-order model and Langmuir isotherm fitted better by experimental data. According to the obtained thermodynamic parameters such as ΔG◦ (free energy), ΔH◦ (enthalpy) and ΔS◦ (entropy) the adsorption was spontaneous, favorable and naturally exothermic process. Moreover, reusability of the synthesized sorbent evaluated in five continuous cycles of sorption–elution process that slight decrease was observed.
